Fractional distillation – principle and methodology MCQs With Answer

Fractional distillation – principle and methodology MCQs With Answer

by

Fractional distillation is a core separation technique in pharmaceutical sciences that relies on vapor-liquid equilibrium to separate components with close boiling points. B. Pharm students should master the principle, column design, reflux ratio, theoretical plates, and practical methodology for solvent purification, solvent recovery, and removal of residual solvents in drug manufacturing. Understanding azeotropes, vacuum and steam distillation, packing vs tray columns, and efficiency calculations (HETP, Murphree efficiency, McCabe-Thiele) is essential for quality control and regulatory compliance. This guide focuses on principle and methodology MCQs tailored for pharmacy curricula to reinforce theory and practical troubleshooting. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the fundamental principle driving fractional distillation?

  • Separation by differences in solubility
  • Separation by differences in vapor pressure and boiling points
  • Separation by differences in magnetic susceptibility
  • Separation by differences in density only

Correct Answer: Separation by differences in vapor pressure and boiling points

Q2. What role does the fractionating column play in fractional distillation?

  • Acts purely as a condenser
  • Provides multiple vapor-liquid equilibrium stages to improve separation
  • Lowers the boiling point of the mixture
  • Removes non-volatile impurities chemically

Correct Answer: Provides multiple vapor-liquid equilibrium stages to improve separation

Q3. Which parameter defines the ratio of condensed liquid returned to the column to the distillate collected?

  • Relative volatility
  • Reflux ratio
  • HETP
  • Theoretical plates

Correct Answer: Reflux ratio

Q4. Relative volatility (α) greater than 1 indicates:

  • Components form an azeotrope
  • Component A is more volatile than component B
  • No separation is possible
  • Component A is less volatile than component B

Correct Answer: Component A is more volatile than component B

Q5. Which law describes vapor pressures of components in an ideal binary mixture?

  • Henry’s law
  • Raoult’s law
  • Le Chatelier’s principle
  • Boyle’s law

Correct Answer: Raoult’s law

Q6. Azeotropes are problematic because they:

  • Have infinite relative volatility
  • Boil at a constant composition and cannot be separated by simple distillation
  • Are only found in ternary mixtures
  • Always have higher boiling points than both pure components

Correct Answer: Boil at a constant composition and cannot be separated by simple distillation

Q7. Which technique is commonly used to break an azeotrope?

  • Simple distillation at atmospheric pressure
  • Extractive or azeotropic distillation and pressure-swing distillation
  • Magnetic separation
  • Crystallization only

Correct Answer: Extractive or azeotropic distillation and pressure-swing distillation

Q8. The number of theoretical plates in a column represents:

  • The physical number of trays only
  • Equivalent ideal vapor-liquid equilibrium stages achieving a given separation
  • The total height of the column in meters
  • The volume of reboiler

Correct Answer: Equivalent ideal vapor-liquid equilibrium stages achieving a given separation

Q9. The McCabe-Thiele method is primarily used to:

  • Design vacuum pumps
  • Graphically determine the number of theoretical stages for binary distillation at constant reflux
  • Measure column diameter
  • Calculate boiling point elevation due to salts

Correct Answer: Graphically determine the number of theoretical stages for binary distillation at constant reflux

Q10. Which of the following increases column separation efficiency for a given feed?

  • Decreasing reflux ratio
  • Increasing reflux ratio (within practical limits)
  • Removing the condenser
  • Using larger boiling stones

Correct Answer: Increasing reflux ratio (within practical limits)

Q11. HETP stands for:

  • Heat Equivalent to Theoretical Pressure
  • Height Equivalent to a Theoretical Plate
  • High Efficiency Thermal Performance
  • Hydrostatic Equilibrium Table Parameter

Correct Answer: Height Equivalent to a Theoretical Plate

Q12. Murphree plate efficiency measures:

  • The actual separation per plate relative to an ideal plate
  • The heat transfer coefficient of a tray
  • The vapor velocity in the column
  • The reboiler duty per mole

Correct Answer: The actual separation per plate relative to an ideal plate

Q13. For thermolabile pharmaceutical compounds, which distillation technique is preferred?

  • Simple atmospheric distillation
  • Vacuum (reduced pressure) distillation
  • Fractional distillation at high reflux
  • Steam distillation at high temperature

Correct Answer: Vacuum (reduced pressure) distillation

Q14. Steam distillation is particularly useful when:

  • Both components are volatile and miscible
  • One component is non-volatile and the other is ionic
  • Separating high-boiling immiscible or heat-sensitive compounds like essential oils
  • Working with cryogenic liquids

Correct Answer: Separating high-boiling immiscible or heat-sensitive compounds like essential oils

Q15. Packing materials in a column (e.g., Raschig rings) are chosen to:

  • Reduce liquid-vapor contact area
  • Increase surface area for vapor-liquid contact and reduce pressure drop
  • Act as catalysts for decomposition
  • Prevent condensation entirely

Correct Answer: Increase surface area for vapor-liquid contact and reduce pressure drop

Q16. What does the reboiler do in a distillation setup?

  • Condenses the overhead vapor into liquid
  • Provides heat to vaporize the bottom liquid, generating rising vapors
  • Measures column pressure
  • Pumps reflux back to the top

Correct Answer: Provides heat to vaporize the bottom liquid, generating rising vapors

Q17. In a binary distillation, the vapor phase is generally richer in:

  • The less volatile component
  • The more volatile component
  • The non-volatile impurities
  • The heavier component by density

Correct Answer: The more volatile component

Q18. Which statement about minimum reflux ratio is correct?

  • It is achieved at total reflux
  • It is the lowest reflux ratio at which the desired separation is theoretically possible
  • It is always zero
  • It equals the number of theoretical plates

Correct Answer: It is the lowest reflux ratio at which the desired separation is theoretically possible

Q19. Flooding in a column refers to:

  • Excessive vapor velocity causing liquid to be carried upward and prevent proper contact
  • Overflow of the reboiler only
  • Purging of column internals with steam
  • Intentional oversupply of reflux to increase purity

Correct Answer: Excessive vapor velocity causing liquid to be carried upward and prevent proper contact

Q20. Which tray type provides good operational flexibility and is widely used in distillation columns?

  • Bubble-cap trays
  • Sieve trays
  • Packless trays
  • Static trays

Correct Answer: Bubble-cap trays

Q21. The overhead condenser in a distillation setup primarily:

  • Vaporizes the bottom liquid
  • Condenses vapor into liquid for collection or reflux
  • Measures feed composition
  • Increases pressure in the column

Correct Answer: Condenses vapor into liquid for collection or reflux

Q22. Which of the following increases when a non-volatile solute is added to a solvent?

  • Vapor pressure of solvent
  • Boiling point of the solution (boiling point elevation)
  • Relative volatility becomes infinite
  • Solution becomes ideal and follows Raoult’s law exactly

Correct Answer: Boiling point of the solution (boiling point elevation)

Q23. In pharmaceutical solvent recovery, which factor is crucial for selecting fractional distillation parameters?

  • Color of the solvent only
  • Thermal stability and boiling point differences of components
  • Magnetic properties of solvents
  • Ambient humidity only

Correct Answer: Thermal stability and boiling point differences of components

Q24. The feed stage location in a distillation column affects:

  • Only the condenser efficiency
  • Required number of stages and column load distribution
  • The definition of azeotropes
  • The chemical composition of feed

Correct Answer: Required number of stages and column load distribution

Q25. Which of these models is commonly used to predict VLE for non-ideal mixtures in distillation design?

  • Nernst equation
  • NRTL (Non-Random Two-Liquid) model
  • Ideal gas law only
  • Henderson-Hasselbalch equation

Correct Answer: NRTL (Non-Random Two-Liquid) model

Q26. Azeotropic distillation can use an added component called an entrainer to:

  • Increase column pressure only
  • Alter relative volatility to allow separation of the original components
  • React chemically to form a new drug
  • Cool the column internals

Correct Answer: Alter relative volatility to allow separation of the original components

Q27. Which operational problem is indicated by high liquid holdup and poor product purity?

  • Insufficient reflux
  • Entrainment and flooding
  • Excessive condenser duty
  • Low feed temperature only

Correct Answer: Entrainment and flooding

Q28. The dew point of a mixture in distillation refers to:

  • The temperature at which the first vapor forms on heating
  • The temperature at which the first liquid forms from vapor on cooling
  • The maximum allowable column temperature
  • The boiling point of pure component A

Correct Answer: The temperature at which the first liquid forms from vapor on cooling

Q29. In fractional distillation, increasing column height (or number of trays) generally:

  • Decreases separation efficiency
  • Improves separation by providing more equilibrium stages
  • Makes the column unusable for pharmaceutical solvents
  • Reduces reflux requirement to zero

Correct Answer: Improves separation by providing more equilibrium stages

Q30. Which is a common packing material used to reduce HETP in packed columns?

  • Sand
  • Raschig rings or structured packing
  • Wood chips
  • Paper towels

Correct Answer: Raschig rings or structured packing

Q31. What is the main advantage of vacuum distillation in pharmaceutical manufacturing?

  • It increases boiling points to speed reaction
  • It allows distillation at lower temperatures to protect thermolabile substances
  • It eliminates the need for condensers
  • It increases relative volatility automatically

Correct Answer: It allows distillation at lower temperatures to protect thermolabile substances

Q32. Head temperature monitoring during distillation helps to:

  • Control condenser pressure only
  • Detect composition changes in the distillate and track cut points
  • Measure vapor density directly
  • Calculate exact number of theoretical plates

Correct Answer: Detect composition changes in the distillate and track cut points

Q33. Which effect does increasing vapor velocity have on a tray column under constant conditions?

  • Reduced chance of entrainment and improved separation
  • Higher pressure drop and potential for entrainment/flooding
  • Lower tray efficiency by mechanical failure only
  • It has no effect

Correct Answer: Higher pressure drop and potential for entrainment/flooding

Q34. Extractive distillation differs from azeotropic distillation in that it:

  • Uses an entrainer that forms a low-boiling azeotrope with one component
  • Adds a high-boiling solvent (entrainer) to change relative volatility without forming a new azeotrope
  • Requires cryogenic temperatures
  • Is only used for inorganic salts

Correct Answer: Adds a high-boiling solvent (entrainer) to change relative volatility without forming a new azeotrope

Q35. Which statement best describes entrainment in distillation?

  • Liquid droplets carried with vapor into the overhead causing contamination
  • Complete condensation of vapor in the reboiler
  • Formation of non-volatile residue only
  • Trapping of gases in the liquid phase permanently

Correct Answer: Liquid droplets carried with vapor into the overhead causing contamination

Q36. Boiling chips are added to distillation flasks to:

  • Increase vapor pressure
  • Prevent bumping by promoting smooth boiling
  • Act as a catalyst for separation
  • Enhance azeotrope formation

Correct Answer: Prevent bumping by promoting smooth boiling

Q37. For a binary mixture with a large boiling point difference (>25°C), which method is usually sufficient?

  • Fractional distillation with many trays
  • Simple (single-stage) distillation
  • Extractive distillation always
  • Membrane separation only

Correct Answer: Simple (single-stage) distillation

Q38. The reflux drum in a distillation plant is used to:

  • Store bottoms product
  • Collect condensed overhead liquid and split it between distillate product and reflux to the column
  • Heat the feed before entering the column
  • Measure column height

Correct Answer: Collect condensed overhead liquid and split it between distillate product and reflux to the column

Q39. Pressure-swing distillation exploits the fact that:

  • Azeotrope compositions change with pressure and can allow separation by changing pressure
  • Pressure does not affect azeotropes
  • Relative volatility is constant regardless of pressure
  • Only temperature matters for azeotrope formation

Correct Answer: Azeotrope compositions change with pressure and can allow separation by changing pressure

Q40. In pharmaceutical quality control, distillation is used to:

  • Remove heavy metals from APIs
  • Determine and remove residual solvents and purify solvents for reuse
  • Sterilize injectable products directly
  • Change the molecular structure of drugs

Correct Answer: Determine and remove residual solvents and purify solvents for reuse

Q41. Which measurement gives an estimate of column efficiency per unit height?

  • Reflux ratio
  • HETP (Height Equivalent to a Theoretical Plate)
  • Boiling point of feed
  • Number of feed trays

Correct Answer: HETP (Height Equivalent to a Theoretical Plate)

Q42. When designing a distillation column for pharmaceutical solvents, which safety consideration is important?

  • Use of non-conductive materials only
  • Managing flammable solvent vapors (inerting, grounding, explosion-proof equipment)
  • Eliminating condensers to reduce risk
  • Avoiding vacuum at all costs

Correct Answer: Managing flammable solvent vapors (inerting, grounding, explosion-proof equipment)

Q43. If relative volatility approaches 1, separation by conventional distillation becomes:

  • Easier and requires fewer stages
  • Very difficult and may require azeotropic or extractive methods
  • Unaffected
  • Improved by simple distillation

Correct Answer: Very difficult and may require azeotropic or extractive methods

Q44. The term “total reflux” refers to:

  • All condensed distillate being returned to the column with no product withdrawal
  • Zero reflux and maximum distillate withdrawal
  • Reflux equal to the feed flow rate
  • Reflux only in the reboiler

Correct Answer: All condensed distillate being returned to the column with no product withdrawal

Q45. Which equipment change would most directly reduce HETP in a packed column?

  • Using larger diameter packing rings
  • Switching to structured packing with higher surface area
  • Increasing column pressure only
  • Removing the reboiler

Correct Answer: Switching to structured packing with higher surface area

Q46. The liquid composition on a tray is usually determined by:

  • Equilibrium between the ascending vapor and descending liquid on that tray
  • Only the feed composition
  • The material of the tray
  • The ambient humidity

Correct Answer: Equilibrium between the ascending vapor and descending liquid on that tray

Q47. Which control parameter is critical for maintaining desired distillate purity?

  • Reflux ratio and reboiler duty (column energy balance)
  • Color of the packing
  • Number of operators in the distillation room
  • Ambient lighting

Correct Answer: Reflux ratio and reboiler duty (column energy balance)

Q48. In a ternary mixture, separating all three components by fractional distillation typically requires:

  • A single binary distillation step
  • Sequential distillation cuts or multiple columns/sections designed for multicomponent separation
  • No reflux at all
  • Only steam distillation

Correct Answer: Sequential distillation cuts or multiple columns/sections designed for multicomponent separation

Q49. Which phenomenon can indicate column tray damage or weeping?

  • Stable overhead composition
  • Low efficiency and liquid leakage through perforations at low vapor rates
  • Improved separation at low vapor rate
  • Increase in HETP performance

Correct Answer: Low efficiency and liquid leakage through perforations at low vapor rates

Q50. For safe distillation of peroxide-forming solvents, a recommended practice is to:

  • Distill to dryness to remove peroxides
  • Test and remove peroxides before distillation or use inhibitors and avoid concentrating peroxides
  • Always distill at highest temperature
  • Mix with strong oxidizers to neutralize peroxides

Correct Answer: Test and remove peroxides before distillation or use inhibitors and avoid concentrating peroxides

Leave a Comment